Electrochemical cells produce a specific voltage which is dependent upon the materials making up the cell. Within fairly narrow limits, a desired voltage can be produced from a single cell through the judicious selection of materials. The maximum voltage achievable from a single cell through this method is less than four volts because of the inherent limited difference in voltage potentials of the materials making up the cell. The practical number of available voltages is further reduced by the fact that only a few of the chemical elements can be successfully combined to produce a usable cell. Many theoretically possible cells can not be manufactured successfully for various reasons or they cannot meet the requirements of use under real life conditions.
In the past, batteries producing voltages higher than that of a single cell have been manufactured by connecting a plurality of cells in series. The individual voltages of all the cells are added to produce a final battery voltage which is the sum of the component cell voltages. With the proper choice of component cells, many desired voltages can be achieved.
Another limitation in designing high voltage batteries is the weight and volume of the finished battery. A sixty volt battery requires forty zinc/carbon cells or twenty lithium/sulfur dioxide cells. Such a battery must be large and heavy because of the large number of component cells. Since a part of the weight and volume is for packaging, and not for energy production, a battery with a large number of small cells has a lower energy density than a similarly sized battery with a smaller number of large cells. In order to obtain voltages other than those available from known cells, the user in the past has had to modify his equipment to use the available voltages, or has had to use a voltage converter in combination with his equipment.
Multiple cell batteries have other inherent problems. If one cell fails, the whole battery can fail since the cells are connected in series. This decreases the reliability of the battery. The problem becomes more serious as the number of cells increases. The complete discharge of one cell, before the other cells in a battery discharge, can cause cell reversal. Depending upon the chemical makeup of the cell, it may cause damage to the battery, and to the device powered by the battery. This can be a serious problem with any type of cell, but especially with cells containing lithium, in which cell reversal is suspected as a cause of serious problems. Obviously, the fewer the number of cells used in a battery, the less the chance of cell reversal, and of battery failure.
A problem related to cell reversal, but not limited to multicell batteries, is that of battery recharging. Many electrical devices, under certain circumstances, can produce a voltage of polarity opposite to that of the cell. Such reverse voltage will tend to recharge the cell with an undesirable effect which is similar to the reversal of a cell in the battery.